1. Field of the Invention
The present invention relates to a pigment ink for ink jet recording, excellent in dispersion stability and storage stability, and exhibiting a high ejection stability, even in an ink jet recording process in which the ink is ejected by the application of thermal energy as ejection energy. It further relates to a pigment ink capable of obtaining an image having a sufficient scratch resistance, in case of printing on a glossy paper as the ink jet recording medium.
2. Description of the Related Art
An ink for ink jet recording, employed in the ink jet recording process, is for example a dye-based ink constituted of water as a principal component and a colorant which is a dye soluble in the water. The dye-based ink has advantages of being excellent in the prolonged storage, not easily causing a nozzle clogging at the ejection, and providing a print excellent in color saturation and transparency. On the other hand, it is associated with drawbacks of being deficient in the weather resistance and the water resistance of the obtained print.
Therefore, attention is recently paid to a pigment ink utilizing a pigment as the colorant for the purpose of improving the weather resistance of the image in particular. The pigment, which is not soluble in water, has to be dispersed in an ink solvent, in order to be used as an ink. It may however be difficult to maintain the dispersed state of the pigment in the ink in an optimum state over a prolonged period, without causing aggregation or sedimentation. Also in the case of a process utilizing an electrothermal converting element as the means for supplying the aforementioned ejection energy, as a liquid droplet is ejected by a bubble generation occurring on providing the ink with thermal energy, it can be anticipated that the thermal energy at the bubble formation causes a rapid change in the properties of the ink. Therefore, particularly in case of ejecting a pigment ink of a low dispersion stability, stable ejection characteristics may not be achieved for example because of deposits accumulated in the ejection nozzle.
In order to solve such problems, there is proposed to use, as a dispersant, a graft copolymer in which either one of a main chain unit and a side chain unit is a hydrophilic segment and the other is a hydrophobic segment (Japanese Patent Application Laid-Open No. H10-087768). In such structure, the hydrophobic segment constituting the graft copolymer fulfills a function of being physically adsorbed, through a hydrophobic interaction, on the hydrophobic pigment particles, while the hydrophilic segment performs a function of spreading in water as the ink solvent, thereby providing the pigment particle with a repulsive force. In the pigment ink utilizing such graft copolymer in which the hydrophobic segment and the hydrophilic segment are clearly separated, the dispersion stability of the pigment and the long-term storage stability and the ejection stability of the pigment ink are improved by the effective expression of the respective functions.
On the other hand, in the recording medium for ink jet recording, in order to enable a recording of high image quality with a dye-based ink, there has been employed a recording medium provided with an ink receiving layer, having many air gaps on the surface, thereby improving the ink absorbing property. The dye-based ink, in which the dye molecules are dissolved in the ink, can be promptly absorbed in the ink receiving layer to provide a record of a high image quality. However, in the case of printing with a pigment ink on such recording medium, the pigment particles are fixed on the surface of the recording medium since most of the pigment particles dispersed in the ink are larger than the air gaps in the ink receiving layer. As a result, in the case that the mutual cohesion of the pigment particles or the adhesion between the recording medium and the recorded image is not sufficient, the image becomes inferior in the scratch resistance and causes a problem that the record is scraped off even by a slight scratching on the surface.
In order to solve such problem, improvements have been made by adding a water-soluble resin, an emulsion or a latex to the ink (see Japanese Patent Application Laid-Open Nos. H05-263029 and 2001-106951). However, in the case of adding an ordinary water-soluble resin to the ink, there may result a problem of an unstable ink ejection or a clogging due to an increase in the ink viscosity, whereby the ink becomes inferior in the ink jet characteristics. On the other hand, in the case of adding an emulsion or a latex, an ink ejection failure tends to occur because of foaming by the emulsion or an increase in the ink viscosity, or a faulty ink ejection or a defective print tends to occur because of sticking of the emulsion in the nozzle interior or on the nozzle face.
Also proposed is an improvement on the recording medium itself, for example by defining the surface roughness (Japanese Patent Application Laid-Open No. 2004-114535). However, this method necessitates a particular recording medium, and a satisfactory record cannot be obtained by the conventional recording medium for ink jet recording.